Chlorine dioxide has found wide use as a bleaching agent in pulp and paper production, as a disinfectant in water treatment/purification, and a number of other uses because of its high oxidizing power. There are a number of chlorine dioxide generator systems and processes available in the marketplace. Most of the very large scale generators utilize a chlorate salt, a chloride ion source or reducing agent, and a strong acid such as sulfuric or hydrochloric acid. Reducing agents which have been used include methanol or other organic compounds, sulfur, sulfur dioxide or other sulfur-oxygen species having a sulfur valence of less than +6, and carbon monoxide among others. When organic compounds are used, unreacted volatile organics including formic acid are present in the product gas.
When using sulfuric acid or sulfur containing reducing agents, the sulfate produced accumulates as a waste product. In addition, prior art processes for the production of chlorine dioxide from chlorate salts require an excess of the acid used. This acid is slowly neutralized by the accumulation of alkali metal or alkaline earth metal ions that enter the process with the chlorate salt. The accumulation of salts must be removed as a waste stream, either liquid or solid, in every process currently practised commercially. Also present in the acidic salt slurry or solution formed in the generator are trace amounts of metals such as transition metals which are present in the reactants or leached or extracted from the apparatus or catalysts used. Accumulation of these impurities and by-products requires periodic shutdowns of the generator for boil-outs or catalyst changes. Thus a process which reduces the amount of a by-product salt, such as sodium chloride or sodium sulfate, and removes metallic impurities produced while efficiently generating chlorine dioxide is commercially desirable.
In U.S. Pat. No. 4,806,215, Twardowski describes a method for producing chlorine dioxide in which the chlorine dioxide is produced in a generator by the reaction of sodium chlorate and hydrochloric acid. After separating chlorine dioxide gas containing chlorine, the remaining sodium chloride solution is fed to the central compartment of a three-compartment cell to form an acidified liquor containing hydrochloric acid. The sodium ions pass through a cation exchange membrane to the cathode compartment to form sodium hydroxide. Sodium hydroxide and the acidified sodium chloride solution are returned to the chlorine dioxide generator. The process accumulates acidified sodium chloride which requires disposal.
To avoid the formation of an acidic alkali metal salt, it has been proposed that chlorine dioxide be prepared from chloric acid. Chloric acid is, however, not commercially available.
Chloric acid preparation has been taught, for example, in U.S. Pat. No. 3,810,969 issued May 14, 1974 to A.A. Schlumberger. Schlumberger teaches a process for producing chloric acid by passing an aqueous solution containing from 0.2 gram mole to 11 gram moles per liter of an alkali metal chlorate such as sodium chlorate through a selected cationic exchange resin at a temperature from 5.degree. to 40.degree. C. The process produces an aqueous solution containing from 0.2 gram mole to about 4.0 gram moles of HClO.sub.3. This process requires the regeneration of the cationic exchange resin with acid to remove the alkali metal ions and the treatment or disposal of the acidic salt solution.
K.L. Hardee et al, in U.S. Pat. No. 4,798,715 issued Jan. 17, 1989, describe a process for chlorine dioxide which electrolyzes a chloric acid solution produced by passing an aqueous solution of an alkali metal chlorate through an ion exchange resin. The electrolyzed solution contains a mixture of chlorine dioxide and chloric acid which is fed to an extractor in which the chlorine dioxide is stripped off. The ion exchange resin is regenerated with hydrochloric acid and an acidic solution of an alkali metal chloride formed.
Each of the above processes produces a fixed amount and type of by-product salt.
M. Lipsztajn et al., in U.S. Pat. No. 4,915,927 issued Apr. 10, 1990, teach an electrolytic-dialytic process for producing chloric acid and sodium hydroxide from sodium chlorate. The sodium chlorate is fed to the central compartment of a three-compartment cell. Chlorate ions are transferred through an anion-exchange membrane to the anode compartment and sodium ions are passed through a cation-exchange membrane to the cathode compartment.
R. M. Berry et al., (PCT WO 90/10733, Sep. 20, 1990) also teach an electrolytic-dialytic process for producing chloric acid and sodium hydroxide from sodium chlorate. The electrochemical cell uses a three compartment water splitting system comprised of repeating cationic, anionic, and bipolar membranes. An alkali metal chlorate solution is fed into a center salt compartment bounded by an anion membrane and cation membrane, between adjoining acid and base compartments which adjoin the bipolar membranes. The alkali metal ions move under the applied direct current from the salt compartment cation membrane into the acid compartment to form the alkali metal hydroxide, and the chlorate ions move through the salt compartment anion membrane into the acid compartment to form chloric acid.
Also present in the acidic salt slurry or solution formed in the generator are trace amounts of metals such as transition metals which are present in the reactants or leached or extracted from the apparatus or catalysts used. Accumulation of these impurities and by-products requires periodic shutdowns of the generator for boil-outs or catalyst changes.